Polyurethane adhesives have been popular for adhering fiber-reinforced plastic substrates to each other and to other objects such as metal fasteners and brackets due to the desirable chemical, heat, and aging resistance of crosslinked polyurethanes. These polyurethanes are specially formulated to have high tensile strength and to resist accelerated aging during painting and oven baking cycles to which the fiber-reinforced substrates are exposed.
Advances in the adhesives occurred with U.S. Pat. No. 3,714,127, to Fabris et al., who disclosed primary polyamines give desirable flow characteristics to the composition so that beads of the adhesive do not run off when applied to vertical surfaces. U.S. Pat. No. 4,876,308 to Melby et al. disclosed that high equivalent ratio of NCO:OH in the prepolymer of from 2 to 20 and a high equivalent ratio of NCO:(OH+NH) in the final adhesive. These higher NCO:(OH+NH) ratios eliminated the need for priming or pretreatment of fiber-reinforced plastic substrates.
Compositions similar to the above-cited patents were used commercially. They were two-component adhesives and the stoichiometry of the reactive species was important to the properties of the adhesives. Metered mixing equipment was needed near the application site to meter and mix the adhesive. The viscosity was controlled by the faster reaction of the isocyanates with the amines forming urea linkages.
It is the object of this invention to prepare a tough one-package polyurethane based structural adhesive composition that solidifies in five minutes or less at 300.degree. F., having high adhesion to molded thermoset fiber reinforced plastic substrates, high cohesive strengths and which does not release volatiles during curing, such as organic solvents and/or blocking agents. These structural adhesives differ from molding compositions by having high cohesive and adhesive strengths.